1. Field of the Invention
The present invention relates to a dual-band antenna and, more particularly, to a bilaterally symmetric dual-band antenna for use in a wireless network device, and a wireless network device having such an antenna.
2. Description of the Prior Art
Please refer to FIG. 1 for a perspective view of a conventional wireless network device 10 in the form of a wireless network card. The wireless network device 10 typically includes: a main body 11, an internal circuit device 12 provided in the main body 11, a connector portion 13 located at one end of the main body 11 and configured for connecting with an external host (not shown), and an antenna signal transceiver portion 14 disposed at the end of the main body 11 that is opposite the connector portion 13. Generally, the antenna signal transceiver portion 14 has a non-metal housing and, when the wireless network device 10 is connected, to an external host, must be exposed outside the external host in order to transmit and receive wireless signals effectively.
FIG. 2 shows a conventional internal circuit device 20 for use in a wireless network device. The internal circuit device 20 includes: a substrate 21, a control circuit 22 provided on the substrate 21, a grounding element 23 covering a predetermined region of the substrate 21, and an antenna unit 24 electrically connected to the control circuit 22. The conventional antenna unit 24 shown in FIG. 2 includes a first antenna 241 and a second antenna 242 which are provided on two lateral sides of the substrate 21, respectively. Moreover, the antennas 241, 242 of the conventional internal circuit device 20 are both designed as printed monopole antennas on the substrate 21. However, due to their limited difference in height in the vertical direction, such printed antennas can only achieve the desired radiation patterns and high gain in the X-Y plane (i.e., along the horizontal directions) by varying the shapes of the first and second antennas 241, 242 but can hardly be improved in terms of gain in the Z direction. With the “vertical stand” design being the design trend of today's wireless network devices in order to save space and lend a sense of modernity and technology to the devices, the low gain of the conventional printed antennas particularly in the Z direction simply cannot satisfy the requirements of wireless network devices having a vertical stand configuration.
For instance, FIG. 3 shows X-Y plane radiation patterns plotted from test results of the first antenna 241 of the conventional printed antenna unit 24 illustrated in FIG. 2. As can be seen in the radiation patterns of FIG. 3, the maximum gain of the first antenna 241 in the vertical direction is merely −15.89 dBi, which is obviously lower than consumers' acceptable lower gain limit (typically −10 dBi) and therefore does not meet the design requirements of high-performance antennas on the market.